Studies by the applicants have shown that human sperm and various fractions of human seminal plasma are capable of inhibiting either spontaneous transformation by lymphocytes or mitogen-induced lymphocyte transformation. It is postulated that the immunosuppressive material in human semen might be essential for the prevention of immunological infertility. The proposal has four major specific aims. The first is to evaluate the immunosuppressive activity of seminal fluids for male partners of infertile couples and from men who have had genital tract infection or vasectomies. The second aim is to characterize and purify the male immunosuppressive factor. The third specific aim is to determine whether antibody dependent cell mediated cytotoxicity is a mechanism involved in human infertility. The final aim is to determine whether a serum immobilizing factor can be released by human lymphocytes.